This invention relates to acquiring a communication channel in a communication system. The invention is especially but not exclusively applicable to ultra-wideband (UWB) radio systems.
A number of standard radio specifications are currently being proposed for UWB radio communications. One example is the MBOA (MultiBand ODFM Alliance) specification. These specifications effectively define a number of radio frequency (RF) channels: in one example, 35 RF channels. Each RF channel is defined by a combination of a frequency band and an RF hopping sequence. In order for two UWB devices to be able to communicate with each other, one must be configured to receive on the same channel as the other is transmitting.
When two UWB devices initially begin to establish a connection with each other they must arrange that they are operating on the same channel. This is achieved in the following way. One of the devices is presumed to be an “established device”, which is operating on a predetermined channel. This may happen because it is already communicating with a third device, or has been doing so. Alternatively, it may happen because the channel acquisition algorithm dictates that a certain device should act either temporarily or permanently as an established device so as to facilitate channel acquisition. The other device (the “searching” device) then scans the channel spectrum until it detects transmissions from the established device.
At the outset the searching device has no knowledge of the channel on which the established device is transmitting. Therefore, the searching device must search each channel in turn until the established device is found. Assuming that the established device is operating on only a single channel and that it is transmitting a beacon on that channel once every 65.536 ms (which is the length of a superframe in at least one proposed system), it may take up to 2.29 seconds (35 channels times 65.536 ms per channel) before the searching device can find the established device.
The established device may support a power-saving mode in which, when it is not in active communication with another device it does not transmit every superframe. If the established device is in a power-saving mode, the searching device may take even longer to find the communication channel. If the established device were to hibernate for 255 superframes at a time it would transmit a beacon on its current channel once every 16.84 seconds. In this situation, to perform a full search of all 35 channels the searching device would have to listen on each channel for at least 16.84 seconds, and so a full search would take 589 seconds, or nearly 10 minutes.
It is possible that neither device could be acting as an established device: both devices could be searching simultaneously. In that case the result would be even worse because as both devices continuously change RF channels they could completely fail to coincide on a common channel. A similar problem may occur if both devices are established and each device is maintaining its current links whilst simultaneously searching for the other device.
There is therefore a need for improving the speed of channel acquisition in such systems.